Developing Contacting Solutions for Mg₂Si_1–<i>x</i>Sn<i>_x</i>-Based Thermoelectric Generators: Cu and Ni₄₅Cu₅₅ as Potential Contacting Electrodes

Abstract

Magnesium silicides can be used for thermoelectric energy conversion as high values of figure of merit zT were obtained for n-type (1.4 at 500 °C) and p-type (0.55 at 350 °C) materials. This, however, needs to be complemented by low resistive and stable contacting to ensure long-term thermogenerator operation and minimize losses. In this study, we selected Cu and Ni₄₅Cu₅₅ as contacting electrodes for their high electrical conductivity, similar coefficient of thermal expansion (CTE), and good adhesion to Mg₂(Si,Sn). Both electrodes were joined to Mg₂Si_0.3Sn_0.7 pellets by hot pressing in a current-assisted press. Microstructural changes near the interface were analyzed using SEM/EDX analysis, and the specific electrical contact resistance <i>r</i>_c was estimated using a traveling potential probe combined with local Seebeck scanning. Good contacting was observed with both electrode materials. Results show low <i>r</i>_c with Cu, suitable for applications, for both n- and p-type silicides (<10 μΩ·cm²), with the occurrence of wide, highly conductive diffusion regions. Ni₄₅Cu₅₅ joining also showed relatively low <i>r</i>_c values (∼30 μΩ·cm²) for n- and p-type but had a less inhomogeneous reaction layer. We also performed annealing experiments with Cu-joined samples at 450 °C for 1 week to investigate the evolution of the contact regions under working temperatures. <i>r</i>_c values increased (up to ∼100 μΩ·cm²) for annealed n-type samples but remained low (<10 μΩ·cm²) for p-type. Therefore, Cu is a good contacting solution for p-type Mg₂(Si,Sn) and a potential one for n-type if the diffusion causing contact property degradation can be prevented.